MATLAB FINANCIAL DERIVATIVES TOOLBOX Manual de usuario Pagina 116

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Since this equals zero, we add it to the objective function that we want to
minimize without affecting the results. Finally, we minimize the quadratic
function without any constraints. Thus, we have to minimize a function, f (so
it can also be solved with methods that we have seen before). So, after
substitution we have:
))r(E)ww()r(Ew)r(EwR(?s)ww(w
s)ww(wswws)ww(swswf
CBABBAABCBAB
ACBAAABBA
C
BA
BBAA
++++
++++++=
112
1221
222222
We need to solve this problem to find the weights of the three assets in the
portfolio. We thus first find the solution to
A
w ,
B
w and ?. The way to solve
the above problem is now easy. We take the partial derivatives each time in
respect to one of the three unknowns and set them equal to zero, and we will
derive a set of three equations with three unknowns. The first is the partial
derivative of f w.r.t
A
w , the second w.r.t
B
w and the third w.r.t the
Langrangean multiplier ?.
The first equation gives:
0
224222222
22
=+
++++
)r(E?)r(E?
sws)ww(sws)ww(sw
C
A
BCBACBAABB
C
BA
AA
The second equation gives:
0
422222222
22
=+
++++
)r(E?)r(E?
s)ww(swsws)ww(sw
C
B
BCBAACAABA
C
BA
BB
The third equation gives:
01 = )r(E)ww()r(Ew)r(EwR
CBABBAA
To work with this case study, assume the following data:
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